1. Field of the Invention
The present invention relates to a piston disposed in an automatic transmission, and more particularly to a piston including a sleeve or cylindrical portion.
2. Discussion of Related Art
As a piston to be disposed in an automatic transmission, there is known a piston of a type including a sleeve or cylindrical portion, which has a relatively small radial dimension and requires a relatively small space for its installation in the automatic transmission. JP-2000-81119A, discloses an example of such a piston, which is disposed radially outwardly of a rotary drum. The cylindrical portion of the piston has a cutout formed at a predetermined circumferential position thereof. A speed sensor fixed to the casing of the automatic transmission extends through the cutout, for detecting the rotating speed of the rotary drum. The required axial dimension of the automatic transmission wherein the speed sensor is positioned within the axial length of the cylindrical portion of the piston can be reduced as compared with that of an automatic transmission wherein the speed sensor is positioned outside the axial length of the cylindrical portion of the piston.
Where the piston including the cylindrical portion described above is disposed radially outwardly of one of a pair of mutually meshing gears mounted on respective two parallel shafts, such as a pair of counter gears, the cylindrical portion of the piston must have a cutout formed in a circumferential part thereof corresponding to a circumferential position of the pair of gears at which these gears mesh with each other, so that the cutout permits the mutual meshing engagement of the pair of gears such that a point of the meshing engagement is located within the circumference of the cylindrical portion of the piston, and so that the cutout prevents an interference of the piston with mutually meshing circumferential portions of the gears. It is considered to use this cutout for a speed sensor as disclosed in JP-2000-81119A, for detecting the rotating speed of the gear disposed radially inwardly of the cylindrical portion of the piston. In this case, however, the required size of the cutout is increased to such an extent that permits the speed sensor to extend through the cutout, as well as to permit the mutual meshing engagement of the two gears. The increased size of the cutout gives rise to a risk of reduction of strength and rigidity of the cylindrical portion of the piston, and functional deterioration of the automatic transmission, such as seizure of a clutch operated by the piston, for example.